1. Field of the Invention
The present invention relates to a device for direct or indirect application of liquid or viscous medium onto a moving material web, especially a paper or cardboard web, and, more particularly, to a metering gap that is formed between an inlet side lip and an outlet side lip of the applicator device.
2. Description of the Related Art
A generic applicator device is known from EP-A 0701022. In this publication a so-called “open-jet nozzle” is disclosed, whereby a free and unsupported stream that is ejected under pressure from the delivery or application nozzle impacts the moving material web directly or indirectly.
An additional generic applicator device is described, for example in publication DE 10012345. This relates to a so-called “Curtain Coater”, a curtain-type applicator device, whereby the coating medium reaches the surface that is to be coated, essentially due to gravity.
Both types of applicator devices are equipped with a manifold for the coating medium. From the manifold the medium travels through openings into an inlet gap and from there into the metering gap that is embodied by an applicator nozzle from where the medium subsequently emerges. The metering gap is formed by two nozzle lips. By adjusting one or both lips, the outlet width can be varied.
In direct application of the medium, a material web travels past the delivery nozzle where it is treated directly with the medium. In doing so, the material web travels over at least one supporting surface, for example over a backing roll.
In indirect application of the medium the coating medium is first applied to the surface of an applicator element (for example an applicator roll or a continuous revolving belt) in order to be transferred from the applicator element to the material web in a nip through which the material web travels.
Of the two lips forming the metering gap, the lip that is located on the side of the metering gap on which the material web travels toward the application device is referred to as the inlet side lip. Consequently, the lip that is located on the side of the metering gap on which the material web travels away from the application device is referred to as the outlet side lip.
All these referenced components are full machine width (in other words, extending across the entire width of the paper or cardboard web manufacturing or converting machine). These components are essentially adapted to the width of the web that is to be treated, whereby currently a width of approximately 10 m and more is no longer rare. An additional factor is that with ever increasing demands for higher machine speeds, a uniform application of the medium becomes increasingly more difficult. In addition, long extended components of this type are susceptible to deflection.
Good application results are achieved with nozzle applicator devices of this type. These applicator devices, or their nozzles have however the disadvantage that they are very susceptible to clogging at the delivery area of the machine-wide metering gap. This gap is very narrow and has an opening of only a few millimeters, especially only approximately 0.8 mm. Over a period of time small particles, such as contaminants and particles of the application medium that commonly have a temperature of 60° C. or hotter may adhere to that side of the metering gap that is in contact with the medium and may harden there, so that the application across the entire width of the material web is no longer sufficiently uniform, thereby reducing the coating quality. Because of this, very expensive clean-up procedures are necessary in order to directly remove the deposits (“caking”) either mechanically, or through hydraulic means. Often, the machine must even be shut down for this, imposing considerable limits upon machine availability, as well as causing production losses and related high costs.
In some instances attempts are made to cool these areas from the outside. These measures however, are not successful since the cooling effect for the critical area is not sufficient.